Background: The local administration of μ-opioid receptor (MOR) agonists attenuates neuropathic pain but the precise mechanism implicated in this effect is not completely elucidated. We investigated if nitric oxide synthesized by neuronal (NOS1) or inducible (NOS2) nitric oxide synthases could modulate the local antiallodynic effects of morphine through the peripheral nitric oxide-cGMP-protein kinase G (PKG)-ATP-sensitive K+ (KATP) channels signaling pathway activation and affect the dorsal root ganglia MOR expression during neuropathic pain.

Results: In wild type (WT) mice, the subplantar administration of morphine dose-dependently decreased the mechanical and thermal allodynia induced by the chronic constriction of the sciatic nerve (CCI), which effects were significantly diminished after their co-administration with different subanalgesic doses of a selective NOS1 (N-[(4S)-4-amino-5-[(2-aminoethyl)amino]pentyl]-N'-nitroguanidine tris(trifluoroacetate) salt; NANT), NOS2 (L-N(6)-(1-iminoethyl)-lysine; L-NIL), L-guanylate cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; ODQ), PKG ((Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate; Rp-8-pCPT-cGMPs) inhibitor or a KATP channel blocker (glibenclamide). The evaluation of the expression of MOR in the dorsal root ganglia from sham-operated and sciatic nerve-injured WT, NOS1 knockout (KO) and NOS2-KO mice at 21 days after surgery demonstrated that, although the basal mRNA and protein levels of MOR were similar between WT and both NOS-KO animals, nerve injury only decreased their expression in WT mice.

Conclusions: These results suggest that the peripheral nitric oxide-cGMP-PKG-KATP signaling pathway activation participates in the local antiallodynic effects of morphine after sciatic nerve injury and that nitric oxide, synthesized by NOS1 and NOS2, is implicated in the dorsal root ganglia down-regulation of MOR during neuropathic pain.

Figure 1: Antiallodynic effects of morphine. Effects of the subplantar administration of different doses (logarithmic axis) of morphine or vehicle on the mechanical (A) and thermal allodynia (B) induced by CCI in the ipsilateral paw of WT mice at 21 days after surgery. Morphine was administered 20 min before starting behavioral testing. Data are expressed as mean values of maximal possible effect (%) for mechanical allodynia and inhibition (%) for thermal allodynia ± SEM (5-6 animals for dose). In both tests, for each dose, * P <0.05 and *** P <0.001 denote significant differences between morphine and vehicle treated animals (Student's t test).

Mentions:
The subplantar administration of morphine into the ipsilateral paw dose-dependently inhibited the mechanical (Figure 1A) and thermal (Figure 1B) allodynia induced by the chronic constriction of the sciatic nerve. Thus, the mechanical and thermal antiallodynic effects produced by high doses of morphine in the ipsilateral paw of sciatic nerve-injured WT mice were significantly higher than those obtained in their corresponding vehicle treated groups (P <0.05; Student's t test). Moreover, analyzing the ED50 values our data showed that the potency of morphine on the inhibition of mechanical, 194.9 nmol (148.7-255.9) and thermal sensitivity, 225.9 nmol (191.0-267.1) induced by sciatic nerve injury was very analogous.

Figure 1: Antiallodynic effects of morphine. Effects of the subplantar administration of different doses (logarithmic axis) of morphine or vehicle on the mechanical (A) and thermal allodynia (B) induced by CCI in the ipsilateral paw of WT mice at 21 days after surgery. Morphine was administered 20 min before starting behavioral testing. Data are expressed as mean values of maximal possible effect (%) for mechanical allodynia and inhibition (%) for thermal allodynia ± SEM (5-6 animals for dose). In both tests, for each dose, * P <0.05 and *** P <0.001 denote significant differences between morphine and vehicle treated animals (Student's t test).

Mentions:
The subplantar administration of morphine into the ipsilateral paw dose-dependently inhibited the mechanical (Figure 1A) and thermal (Figure 1B) allodynia induced by the chronic constriction of the sciatic nerve. Thus, the mechanical and thermal antiallodynic effects produced by high doses of morphine in the ipsilateral paw of sciatic nerve-injured WT mice were significantly higher than those obtained in their corresponding vehicle treated groups (P <0.05; Student's t test). Moreover, analyzing the ED50 values our data showed that the potency of morphine on the inhibition of mechanical, 194.9 nmol (148.7-255.9) and thermal sensitivity, 225.9 nmol (191.0-267.1) induced by sciatic nerve injury was very analogous.

Background: The local administration of μ-opioid receptor (MOR) agonists attenuates neuropathic pain but the precise mechanism implicated in this effect is not completely elucidated. We investigated if nitric oxide synthesized by neuronal (NOS1) or inducible (NOS2) nitric oxide synthases could modulate the local antiallodynic effects of morphine through the peripheral nitric oxide-cGMP-protein kinase G (PKG)-ATP-sensitive K+ (KATP) channels signaling pathway activation and affect the dorsal root ganglia MOR expression during neuropathic pain.

Results: In wild type (WT) mice, the subplantar administration of morphine dose-dependently decreased the mechanical and thermal allodynia induced by the chronic constriction of the sciatic nerve (CCI), which effects were significantly diminished after their co-administration with different subanalgesic doses of a selective NOS1 (N-[(4S)-4-amino-5-[(2-aminoethyl)amino]pentyl]-N'-nitroguanidine tris(trifluoroacetate) salt; NANT), NOS2 (L-N(6)-(1-iminoethyl)-lysine; L-NIL), L-guanylate cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; ODQ), PKG ((Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate; Rp-8-pCPT-cGMPs) inhibitor or a KATP channel blocker (glibenclamide). The evaluation of the expression of MOR in the dorsal root ganglia from sham-operated and sciatic nerve-injured WT, NOS1 knockout (KO) and NOS2-KO mice at 21 days after surgery demonstrated that, although the basal mRNA and protein levels of MOR were similar between WT and both NOS-KO animals, nerve injury only decreased their expression in WT mice.

Conclusions: These results suggest that the peripheral nitric oxide-cGMP-PKG-KATP signaling pathway activation participates in the local antiallodynic effects of morphine after sciatic nerve injury and that nitric oxide, synthesized by NOS1 and NOS2, is implicated in the dorsal root ganglia down-regulation of MOR during neuropathic pain.